Method for preparing metal particles



May 6, 1969 T. w. WILLIAMS METHOD FOR PREPARING METAL PARTICLES Filed April 4, 1966 S M M L H W D L E F W W m T United States Patent 3,442,988 METHOD FOR PREPARING METAL PARTICLES Tom Winfield Williams, Little Rock, Ark., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Apr. 4, 1966, Ser. No. 539,977 Int. Cl. B22d 23/08 US. Cl. 264-8 7 Claims ABSTRACT OF THE DISCLOSURE Particles of molten metal are formed and then passed into foam to at least partially chill them. The particles may be further passed into a cooling liquid whose surface supports the foam. The molten metal particles may be centrifugally east through holes extending through the side wall of a rotating hollow pot. The liquid may be rotated to form a vortex, so that its surface is an annular wall surrounding and spaced from the pot.

This invention relates to improvements in casting solid free-flowing elongated metal particles from molten metal.

The size, shape and surface characteristics of metal particles are partly determined by the inherent properties of the metal itself, such as its rate of oxidation, and the physical characteristics of its oxide. They are also determined by the manner in which the particles are cast, such as the way in which the initial molten droplets are formed, and the means and manner in which they are solidified and collected. For purposes of producing a particle suitable for rolling directly into solid strip, it is desirable to obtain a particle having a smooth surface and an acicular shape having the general proportions of a cucumber (in other words, a shape characterized by a substantially circular cross-section along the central axis of elongation, the maximum circumference being located about midway along said axis, and said cross-section being tapered toward the ends of the particle). The present preferred size of such particles for purposes of rolling is in general finer than about mesh and coarser than about 60 mesh (US. Standard Sieve), but the size can be as fine as 200 mesh, and has no critical upper limit in size for rolling purposes. Such particles are particularly characterized by having a free-flowing characteristic, which can be maintained even when the particles are preheated for rolling.

It is known that such particles are best formed by casting them from a perforated rotating pot into which the molten metal is poured, and from which the molten metal is expelled through the perforations. Individual droplets of molten metal are formed just outside the wall of the pot, and one known system of making the particles is to allow the centrifugally expelled drops to fall through a gaseous atmosphere, usually air, until they are sufficiently solidified and hardened by chilling to permit them to strike a solid collecting surface without distorting the particles so formed. Another known system is to catch the centrifugally expelled particles on the surface of a body of liquid, such as a pool of water, while the particles are at an early or intermediate stage of transition between molten and solid condition. The latter system has the advantage of reducing the length of the free fall of the particles, and consequently the expense of providing a large air pit, but the semi-molten condition of the particles introduces problems of distortion when the particles enter the liquid, in spite of the fact that the liquid provides a more yielding surface than a solid surface would provide. Raising the temperature of the liquid helps to alleviate this problem, and the space requirements can be reduced by rotating the body of liquid so that its normally horizontal surface is caused to extend as an up-raised wall around the spinning pot.

In accordance with the present invention, the flight f hot metal particles is controlled by passing them into an element which chills and decelerates them without distorting their shape. This in turn minimizes or eliminates the tendency of the metal particles to distort in the course of being collected, and has the great advantage of helping to reduce the space which must be provided for making the particles. Also, the invention gives improved control over the process of making the particles. The said element of the invention comprises a multitude of thin films of liquid, such as provided by a layer of foam or froth covering a liquid or solid catching surface.

As used herein, the term foam shall mean a heterogeneous mixture of a gaseous phase in a liquid phase, and shall not include any substance which is primarily solid.

For a better understanding of the invention, and of its other details, objects, and advantages, reference is now made to the accompanying drawings, which show, for purposes of illustration only, present preferred embodiments of the invention. In the drawings:

FIGURE 1 is an elevation view, partially in section, of apparatus according to the invention;

FIGURE 2 is a sectional view taken at 2-2 in FIG- URE 1;

FIGURE 3 is a sectional view taken at 3-3 in FIG- URE 1; and

FIGURE 4 is a representation of elongated particles produced in accordance with the invention.

Referring to the drawings, the illustrated apparatus includes a tank 10 having a cylindrical wall 12, an open upper end, and a bottom end closed by a wall 14. The side wall 12 is secured to fixed supports 16 to hold the tank stationary. A vertical driveshaft 18 extends through a sealed opening in the bottom wall 14 of the tank and is journaled in thrust bearings 20 which are carried in fixed supports 22 below the tank Wall 14. The vertical driveshaft 18 is preferably mounted in the center of the tank, as shown, and a drive pulley 26 is keyed to the lower end of the driveshaft 18. At its upper end, the driveshaft 18 carries a disc 28 which is mounted on the threaded upper end of the shaft, and a cylindrical casting pot 30 is fixed to the top surface of disc 28. The driveshaft 18, the pulley 26, the disc 28 and the pot 30 are concentric with one another and rotatable in the bearings 20 when the pulley 26 is driven by a belt from a conventional motor (not shown). A funnel 23 is held directly above the casting pot 30 by a bridge 24 connected to fixed supports 16. The bridge 24 also supports a trough 25 adapted to discharge into the funnel 23.

The top of the casting pot is open, and a plurality of openings or holes 32 extend through the cylindrical side wall of the pot 30. The openings 32 may extend radially through the side wall but preferably slant backward as illustrated in FIGURE 3 Where the arrow 35 indicates the direction of rotation of the pot. In addition, a heater 36, for example a gas burner, is provided for insertion into the open upper end of the tank 10 near the casting pot 30 for the purpose of pro-heating the pot and maintaining metal which is poured into the pot at the proper casting temperature.

Attached to the bottom surface of disc 28 are impeller tabs 38 formed of flat metal strips bent through a slight angle. One leg 39 of each tab is secured to the disc, while the other leg 40 extends downwardly away from the disc.

A particle discharge funnel '60 connected to the bottom of the tank bottom wall 14 leads into discharge tube 64, and is closed by moveable discharge stopper 62.

In the operation of the apparatus shown in FIGURE 1, a suitable cooling liquid 42, such as water, is poured into the tank until it submerges at least a part of the pot 30. The driveshaft 18 is rotated so that the liquid 42, which is in contact with the disc 28, also rotates. Rotation of the liquid 42 causes it to be forced upwardly along the inside of the tank side wall 12, forming a vortex such that an annular wall 44 of liquid surrounds and is spaced from the pot 30. The distance of the annular wall 44 from the pot 30 is a function of the diameter of disc 28. If it is desired to avoid wetting the pot, the driveshaft is rotated while the liquid is being poured into the tank.

While the vortex is thus created and maintained, impeller tabs 38 agitate the liquid to create bubbles which form a layer of foam or froth 48 on the surface of the liquid wall 44. Preferably detergent is added to enhance this foaming action. The tabs also create a downward circulation of water adjacent the liquid wall 44, as is shown'by arrows 41 in FIGURE 1.

The heater 36 pre-heats the casting pot to approximately the temperature of the molten metal to be poured into it, and in addition keeps the froth away from the casting pot. A stream of molten metal is poured from a furnace (not shown) through the trough 25 and the funnel 23 into the upper end of the pct 30 as shown by arrow 50, and the molten metal is maintained at a level substantially above the uppermost openings 32. The molten metal is then expelled from the openings 32 by the cen trifugal action of the rotating pot, forming particles 52.

The size of the pot and the speed of rotation are such that the molten metal leaving the openings 32 forms elongated particles 52 traveling outwardly to the annular liquid wall 44. As the molten metal particles travel away from the casting pot 30, they pass first through the froth layer 48, and thence through the annular wall 44 and into the liquid 42. Since the initial step of passing the cast particles 52. through foam layer 48 has been found to produce cast particles having improved uniformity as to both form and appearance, it is believed that the froth serves to decelerate, cool and at least partially solidify the molten particles before they impact with the liquid layer, thereby minimizing or preventing the distortion caused by the impact. The liquid 42 further chills the particles to about the temperature of the liquid, which should be a temperature at which the particles are solid and do not adhere together.

Once the particles are disposed in the liquid 42, the circulation caused by the tabs 38 and shown by arrows 41 moves them downwardly and out of the zone where newer particles are entering the liquid, thus decreasing the chance of any particles striking preceding particles while they are still hot enough to weld together. The clockwise rotation of the liquid 42 sweeps the particles from the bottom of the tank 10 into discharge funnel 60. Periodic operation of discharge stopper 62 then removes the particles, together with some cooling liquid, through discharge tube 64, for further processing.

In an illustrative example of the apparatus described above, and its use, the casting pot was made of cast iron with an outside diameter of 2.25 inches and the overall height of 7.45 inches. The inner surface of the pot was formed by drilling a 1.75 inch diameter hole along the pots longitudinal axis to a depth of 4.75 inches, resulting in a wall thickness of 0.25 inch. A total of 160 equally spaced, 0.100 inch diameter holes in the wall were arranged in 20 vertical columns spaced 18 apart, with the centerlines of the 8 holes in each column spaced inch apart. The holes were slanted backward for clockwise pot rotation so that their extended centerlines were 0.66 inch from the pot axis. The tank was made of mild steel and had an inside diameter of 30 inches and an inside depth of 24 inches. The aluminum disc had a diameter of 9 inches and a thickness of inch. The metal tabs had a width of 1 inch and a flattened-out length of 3 inches, were bent through an angle of 20, and were located about A inch from the edge of the disc. A compressed air-butane burner was positioned at an angle of about 45 about 4 to 6 inches from the top of the casting pot. The stroke of the discharge stopper was 6 inches.

Four SO-pound ingots of an aluminum alloy (the major elements being about 91% aluminum and 7% silicon) were charged into a gas-fired tilting-hearth furnace, and the temperature of the furnace increased to about 850 C. After the tank was filled with water to a level even with the top of the casting pot, the shaft, pot and disc were rotated clockwise at 1500 r.p.m., forming a vortex in the bath about 17 inches in diameter. Approximately milliliters of a liquid detergent sold under the trademark Joy by the Procter and Gamble Company were added to the bath. A layer of froth consisting of bubbles approximately inch in diameter was formed on the surface of the bath. The froth had an average vertical thickness of about 5 inches and was spaced radially from the centermost row of casting pot holes by a distance of about 4 inches.

A refractory-lined transfer trough leading from the furnace to a mild steel funnel discharging directly over the casting pot was preheated to a temperature of about 700 C. by a compressed air-natural gas burner. The casting pot was preheated by the compressed air-butane burner to a temperature of about 850 C. When the necessary temperatures were attained, the gas to the furnace was shut off and the furnace tilted, so that the molten aluminum alloy flowed at a temperature between 800 C. and 850 C. into the casting pot and was expelled through the casting pot holes as discrete elongated particles. The rate of metal transfer of 15 pounds per minute was such that the level of molten metal in the casting pot was maintained a minimum of inch above the uppermost casting pot holes.

The cast particles were received in the layer of froth and passed downwardly to the bottom of the tank. Periodic manual operation of the discharge valve allowed the cast particles to fall into a receiving tub. A recirculating pump pumped the water from the receiving tub back to the tank at a rate of about 5 to 7 gallons per minute. A cold water line added water to the receiving tub at a rate sufiicient to keep the temperature of the water in the tank below C.

The flow of metal from the furnace was stopped when a total of pounds of particles had been cast. The total casting time was about 10 minutes.

The cast particles were washed with water and dried with a blast of warm air. The dried particles were weighed and screened. One hundred twenty-five pounds of the particles had the free-flowing smooth acicular shape desirable for direct rolling purposes, and were in the size range of minus 8 to plus 28 mesh (US. Standard Sieve).

Although the above-mentioned alloy is difiicult to cast into the desired particles and is thus advantageously cast into particles by use of foam in accordance with the invention, such foam can be used to cast the desired particles of other alloys of aluminum, and of other metals and their alloys. Furthermore, the particular apparatus and method steps described above are obviously not the only ones which can be used with foam in accordance with the invention, since the deceleration and chilling effect of the foam is applicable in various other systems for casting metal particles, whatever means may be used for projecting the molten particles in the first place, and whatever means may be used for collecting the particles after they have passed into the foam.

What is claimed is:

1. A process of producing metal particles comprising the steps of forming particles of molten metal and passing said particles into foam, which at least partially chills said particles.

2. The process of claim 1 comprising the further step of passing said particles out of the foam into a cooling liquid adjacent the foam.

3. The process of claim 4 wherein the surface of said liquid defines a wall spaced from and extending around said rotating member.

4. The process of claim 1 wherein said molten metal particles are formed by being centrifugally projected from a rotating member having a side wall.

5. The process of claim 4 wherein said molten particles are expelled through holes extending through the side wall of said member.

6. The process of claim 5 wherein said particles are of smooth elongated shape which tapers toward its opposite ends, and are coarser than about 200 mesh.

7. A process of producing solid free-flowing elongated metal particles comprising the steps of: providing a rotatable hollow pot open at its upper end and-having holes extending through its side wall, disposing below said pot a cooling liquid containing a foam-producing agent, agitating said liquid so as to produce a layer of 6 foam on the surface of said liquid, rotating said pot, pouring molten metal in said pot, and expelling by centrifugal force said metal in the form of elongated particles through said holes, into said foam, and thence into said liquid.

References Cited UNITED STATES PATENTS 3,241,948 8/1958 Claiborne et al 264-8 3,272,893 2/1963 Morgensen 264-8 ROBERT F. WHITE, Primary Examiner. J. R. HALL, Assistant Examiner. 

